Solder mask inkjet inks for manufacturing printed circuit boards

ABSTRACT

An inkjet method for producing a solder mask in the manufacture of a Printed Circuit Board uses a solder mask inkjet ink containing at least one photo-initiator, at least one free radical polymerizable compound and at least one RAFT compound as adhesion promoter. A high quality solder mask withstanding the high thermal stress during the soldering process while maintaining excellent physical properties is produced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application of PCT/EP2017/078381, filed Nov. 7, 2017. This application claims the benefit of European Application No. 16198099.0, filed Nov. 10, 2016, which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a solder mask inkjet ink and an inkjet method for manufacturing Printed Circuit Boards.

2. Description of the Related Art

Inkjet printing methods have been proposed to further improve the manufacturing process of Printed Circuit Boards (PCBs).

Inkjet printing methods and inkjet inks have been disclosed for legend printing in for example EP-A 2725075 (Agfa) and for applying an etch resist on a copper surface in for example EP-A 2809735 (Agfa) and EP-A 3000853 (Agfa).

By reducing the complexity and minimizing the waste such inkjet printing methods render the manufacture of PCBs more cost effective.

Also for applying the solder mask, inkjet printing methods have been disclosed in for example EP-A 1543704 (Avecia) and EP-A 1624001 (Taiyo Ink Manufacturing).

Solder masks are permanent protective coatings that perform a number of functions during the fabrication, assembly and end use of PCBs. One of the main purposes of solder mask is to protect the circuitry from interacting with solder during the assembly process. A solder mask's job isn't solely restricted to the solder operation however, as it also helps to protect the laminate, holes and traces from collecting contaminants and from degrading during the service life of the PCB. A solder mask also acts as an insulator of known dielectric property between components and traces of the PCB.

UV curable inks are preferred for the design of solder mask inks as they allow a high crosslinking degree, resulting in excellent chemical resistance and mechanical properties. However, compatibility with the high temperature soldering process, while maintaining all physical properties, is especially challenging.

The solder mask inkjet inks disclosed in EP-A 1543704 include an adhesion promoter, which is preferably a (meth)acrylate containing an acid such as for example 2-carboxyethylacrylate.

In EP-A 1624001 the solder mask inkjet ink comprises a monomer having a (meth)acryloyl group and a thermosetting functional group selected from the group consisting of a hydroxyl group, a carboxyl group, an isocyanate group, an amino group, an imino group, an epoxy group, an oxetanyl group, a mercapto group, a methoxymethyl group, a methoxyethyl group, an ethoxymethyl group, an ethoxyethyl group, and an oxazoline group.

Reversible Addition-Fragmentation Chain Transfer (RAFT) mechanisms have been proposed to reduce stress build up in polymer networks during photo-polymerization using different types of RAFT agents.

The RAFT technology has been disclosed in US 2013096219 (BOWMAN) to reduce shrinkage stress in cross-linked polymerized materials for dental applications.

The unpublished EP-A 16173227.6 (filed Jul. 6, 2016) discloses radiation curable compositions comprising RAFT functionalized photoinitiators.

There is still a need for designing solder mask inkjet inks, which are able to withstand the high thermal stress induced during the soldering process in the manufacture of PCBs.

SUMMARY OF THE INVENTION

Preferred embodiments of the invention provide a solder mask inkjet ink wherewith a high quality solder mask, in particular withstanding the high thermal stress during the soldering process while maintaining excellent physical properties, may be produced.

The advantages and benefits of the invention are realized by the solder mask inkjet ink defined below.

Further objects of the invention will become apparent from the description hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

The term “monofunctional” in e.g. monofunctional polymerizable compound means that the polymerizable compound includes one polymerizable group.

The term “difunctional” in e.g. difunctional polymerizable compound means that the polymerizable compound includes two polymerizable groups.

The term “polyfunctional” in e.g. polyfunctional polymerizable compound means that the polymerizable compound includes more than two polymerizable groups.

The term “alkyl” means all variants possible for each number of carbon atoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc.

Unless otherwise specified a substituted or unsubstituted alkyl group is preferably a C₁ to C₆-alkyl group.

Unless otherwise specified a substituted or unsubstituted alkenyl group is preferably a C₂ to C₆-alkenyl group.

Unless otherwise specified a substituted or unsubstituted alkynyl group is preferably a C₂ to C₆-alkynyl group.

Unless otherwise specified a substituted or unsubstituted aralkyl group is preferably a phenyl or naphthyl group including one, two, three or more C₁ to C₆-alkyl groups.

Unless otherwise specified a substituted or unsubstituted alkaryl group is preferably a C₇ to C₂₀-alkyl group including a phenyl group or naphthyl group.

Unless otherwise specified a substituted or unsubstituted aryl group is preferably a phenyl group or naphthyl group.

Unless otherwise specified a substituted or unsubstituted heteroaryl group is preferably a five- or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms or combinations thereof.

The term “substituted”, in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

Unless otherwise specified a substituted alkyl group, a substituted alkenyl group, a substituted alkynyl group, a substituted aralkyl group, a substituted alkaryl group, a substituted aryl and a substituted heteroaryl group are preferably substituted by one or more constituents selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tertiary-butyl, ester, amide, ether, thioether, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester, sulphonamide, —Cl, —Br, —I, —OH, —SH, —CN and —NO₂.

Manufacture of an Electronic Device

The method of manufacturing an electronic device according to the present invention includes the steps of:

jetting a radiation curable solder mask inkjet ink as described below on a dielectric substrate containing an electrically conductive pattern; and

curing the jetted solder mask inkjet ink.

The electronic device is preferably a Printed Circuit Board.

The radiation curable solder mask inkjet ink may be cured by exposing the ink to actinic radiation, such as electron beam or ultraviolet (UV) radiation.

Preferably the radiation curable inkjet ink is cured by UV radiation, more preferably using UV LED curing.

The method preferably includes a thermal treatment. The thermal treatment is preferably carried out after the curing step.

In a preferred embodiment the thermal treatment is carried out at a temperature from 80° C. to 250° C. The temperature is preferably not less than 100° C., more preferably not less than 120° C. To prevent charring of the solder mask, the temperature is preferably not greater than 200° C., more preferably not greater then 160° C.

The thermal treatment is typically carried out between 15 and 90 minutes.

The purpose of the thermal treatment is to further increase the polymerization degree of the solder mask.

This further polymerization during the thermal treatment may be accelerated by adding radical initiators which promote thermal curing of polymers, such as peroxides and azo compounds, to the solder mask inkjet ink.

The dielectric substrate of the electronic device may be any non-conductive material. The substrate is typically a paper/resin composite or a resin/fibre glass composite, a ceramic substrate, a polyester or a polyimide.

The electrically conductive pattern is typically made from any metal or alloy which is conventionally used for preparing electronic devices such as gold, silver, palladium, nickel/gold, nickel, tin, tin/lead, aluminium, tin/aluminium and copper. The electrically conductive pattern is preferably made from copper.

Radiation Curable Solder Mask Inkjet Ink

The radiation curable solder mask inkjet ink comprises at least one photoinitiator, at least one free radical polymerizable compound, and at least one adhesion promoter as disclosed below.

The solder mask inkjet ink can be cured by e-beam, but is preferably cured by UV light, more preferably by UV light from UV LEDs. The solder mask inkjet ink is thus preferably a UV curable inkjet ink.

For reliable industrial inkjet printing, the viscosity of the radiation curable inkjet inks is preferably no more than 20 mPa·s at 45° C., more preferably between 1 and 18 mPa·s at 45° C., and most preferably between 4 and 14 mPa·s at 45° C.

For good image quality and adhesion, the surface tension of the radiation curable inkjet inks is preferably in the range of 18 to 70 mN/m at 25° C., more preferably in the range of 20 to 40 mN/m at 25° C.

The radiation curable solder mask inkjet ink may further comprise other polymerizable compounds, colorants, polymeric dispersants, a polymerization inhibitor, or a surfactant. The radiation curable solder mask inkjet ink may further contain a flame retardant.

Adhesion Promoter

The adhesion promoter according to the present invention has a structure according to Formula I,

wherein R1 is selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, OR5, SR6 and NR7R8, R2 is selected from the group consisting of hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R2 and R3 may represent the necessary atoms to form 5 to 8 membered ring, R3 is selected from the group consisting of hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, and an electron withdrawing group comprising at least one oxygen carbon double bond or a nitrile group, R4 is selected from the group consisting of an electron withdrawing group comprising at least one oxygen carbon double bond or a nitrile group and a substituted or unsubstituted aryl or heteroaryl group, R5 and R6 are independently selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R7 and R8 are independently selected from the group consisting of hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R7 and R8 may represent the necessary atoms to form a 5 to 8 membered ring, with the proviso that none of R1 to R4 comprises a photoinitiating moiety.

In a preferred embodiment, R1 is selected from the group consisting of OR5, NR7R8 and a substituted or unsubstituted aryl group, OR5 and NR7R8 being more preferred, OR5 being the most preferred.

In a further preferred embodiment the adhesion promoter has a chemical structure according to Formula II,

wherein R9 is selected from the group consisting of OR12, NR13R14 and a substituted or unsubstituted aryl group, R10 is selected from the group consisting of an ester, an amide, a carboxylic acid and a nitrile, R11 represents a substituted or unsubstituted alkyl group, R12 is independently selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R13 and R14 are independently selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R13 and R14 may represent the necessary atoms to form a 5 to 8 membered ring.

In an even more preferred embodiment the adhesion promoter has a chemical structure according to Formula III,

wherein R15 is selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R16 represents a hydrogen or a substituted or unsubstituted alkyl group.

In another preferred embodiment the adhesion promoter has a chemical structure according to Formula IV,

wherein R17 is selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R18 is selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, and an electron withdrawing group comprising at least one oxygen carbon double bond or a nitrile group, R17 and R18 may represent the necessary atoms to form a 5 to 8 membered ring, R19 is selected from the group consisting of an electron withdrawing group comprising at least one oxygen carbon double bond or a nitrile group and a substituted or unsubstituted aryl or heteroaryl group.

In an even further preferred embodiment the adhesion promoter has a chemical structure according to Formula V,

wherein R20 and R21 are independently selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group, R20 and R21 may represent the necessary atoms to form a 5 to 8 membered ring, R22 and R23 are independently selected from the group consisting of a hydrogen and a substituted or unsubstituted alkyl group, R22 and R23 may represent the necessary atoms to form a 5 to 8 membered ring. R24 is selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group and a substituted or unsubstituted aryl or heteroaryl group.

In a more preferred embodiment R22 and R23 represent a lower alkyl group having one to four carbon atoms, a methyl group or an ethyl group being particularly preferred.

In an even further preferred embodiment R24 is selected from the group consisting of a hydrogen atom and a substituted or unsubstituted alkyl group.

Typical adhesion promoters according to the present invention are given in Table 1 without being limited thereto.

TABLE 1

RAFT-1

RAFT-2

RAFT-3

RAFT-4

RAFT-5

RAFT-6

RAFT-7

RAFT-8

RAFT-9

RAFT-10

RAFT-11

RAFT-12

RAFT-13

RAFT-14

RAFT-15

RAFT-16

RAFT-17

The radiation curable solder mask inkjet ink may in addition to the adhesion promoter described above comprise other adhesion promoters, for examples those disclosed in WO2004/028225.

The amount of adhesion promoter in the radiation curable solder mask inkjet ink is preferably between 0.5 and 20 wt %, more preferably between 1 and 15 wt %, most preferably between 2.5 and 10 wt %, relative to the total weight of the inkjet ink.

Free Radical Polymerizable Compounds

The free radical polymerizable compounds may be monomers, oligomers and/or prepolymers.

These monomers, oligomers and/or prepolymers may possess different degrees of functionality. A mixture including combinations of mono-, di-, tri- and higher functional monomers, oligomers and/or prepolymers may be used. The viscosity of the radiation curable inkjet ink may be adjusted by varying the ratio between the monomers and oligomers.

In a preferred embodiment, the monomer, oligomer or polymer includes at least one acrylate group as polymerizable group.

Preferred monomers and oligomers are those listed in paragraphs [0106] to [0115] in EP-A 1911814.

In a preferred embodiment, the radiation curable inkjet ink comprises a monomer containing a vinylether group and an acrylate or methacrylate group. Such monomers are disclosed in EP-A 2848659, paragraphs [0099] to [0104]). A particular preferred monomer containing a vinylether group and an acrylate group is 2-(2-vinyloxyethoxy)ethylacrylate.

In a particular preferred embodiment, the solder mask inkjet ink comprises a free radical polymerizable compound selected from the group consisting of neopentyl-glycol hydroxypivalate diacrylate, isobornyl acrylate, dipropylene glycol diacrylate, trimethylol propane triacrylate, and 2-(vinylethoxy)ethyl acrylate.

Colorants

The radiation curable solder mask inkjet ink may be a substantially colourless inkjet ink, but preferably the radiation curable inkjet includes at least one colorant.

The colorant in the solder mask inkjet ink may be a pigment or a dye, but is preferably a pigment.

A colour pigment may be chosen from those disclosed by HERBST, Willy, et al. Industrial Organic Pigments, Production, Properties, Applications, 3rd edition. Wiley-VCH, 2004, ISBN 3527305769.

Suitable pigments are disclosed in paragraphs [0128] to [0138] of WO2008/074548.

Pigment particles in inkjet inks should be sufficiently small to permit free flow of the ink through the inkjet-printing device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum colour strength and to slow down sedimentation. Most preferably, the average pigment particle size is no larger than 150 nm. The average particle size of pigment particles is preferably determined with a Brookhaven Instruments Particle Sizer BI90plus based upon the principle of dynamic light scattering.

In PCBs, the solder mask typically has a blue or green colour. The blue pigment is preferably one of the phthalocyanine series. Examples of blue pigments are C.I. Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 24 and 60.

Green pigments are generally a mixture of blue and yellow or orange pigments or may be green pigments per se, such as halogenated phthalocyanines, for example copper or nickel brominated phthalocyanine.

In a preferred embodiment, the colorant is present in an amount of 0.2 to 6.0 wt %, more preferably 0.5 to 2.5 wt %, based on the total weight of the radiation curable inkjet ink.

Polymeric Dispersants

If the colorant in the radiation curable solder mask inkjet ink is a pigment, then the inkjet ink preferably contains a dispersant, more preferably a polymeric dispersant, for dispersing the pigment.

Suitable polymeric dispersants are copolymers of two monomers but they may contain three, four, five or even more monomers. The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Copolymeric dispersants preferably have the following polymer compositions:

-   -   statistically polymerized monomers (e.g. monomers A and B         polymerized into ABBAABAB);     -   alternating polymerized monomers (e.g. monomers A and B         polymerized into ABABABAB);     -   gradient (tapered) polymerized monomers (e.g. monomers A and B         polymerized into AAABAABBABBB);     -   block copolymers (e.g. monomers A and B polymerized into         AAAAABBBBBB) wherein the block length of each of the blocks (2,         3, 4, 5 or even more) is important for the dispersion capability         of the polymeric dispersant;     -   graft copolymers (graft copolymers consist of a polymeric         backbone with polymeric side chains attached to the backbone);         and     -   mixed forms of these polymers, e.g. blocky gradient copolymers.

Suitable polymeric dispersants are listed in the section on “Dispersants”, more specifically [0064] to [0070] and [0074] to [0077], in EP-A 1911814.

Commercial examples of polymeric dispersants are the following:

-   -   DISPERBYK™ dispersants available from BYK CHEMIE GMBH;     -   SOLSPERSE™ dispersants available from NOVEON;     -   TEGO™ DISPERS™ dispersants from EVONIK;     -   EDAPLAN™ dispersants from MONZING CHEMIE;     -   ETHACRYL™ dispersants from LYONDELL;     -   GANEX™ dispersants from ISP;     -   DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;     -   DISPONER™ dispersants from DEUCHEM; and     -   JONCRYL™ dispersants from JOHNSON POLYMER.

Photoinitiators and Photoinitiating Systems

The radiation curable solder mask inkjet ink preferably contains at least one photoinitiator, but may contain a photoinitiating system including a plurality of photoinitiators and/or co-initiators.

The photoinitiator in the radiation curable inkjet is preferably a free radical initiator, more specifically a Norrish type I initiator or a Norrish type II initiator. A free radical photoinitiator is a chemical compound that initiates polymerization of monomers and oligomers when exposed to actinic radiation by the formation of a free radical. A Norrish Type I initiator is an initiator which cleaves after excitation, yielding the initiating radical immediately. A Norrish type II-initiator is a photoinitiator which is activated by actinic radiation and forms free radicals by hydrogen abstraction from a second compound that becomes the actual initiating free radical. This second compound is called a polymerization synergist or co-initiator. Both type I and type II photoinitiators can be used in the present invention, alone or in combination.

Suitable photoinitiators are disclosed in CRIVELLO, J. V., et al. Photoinitiators for Free Radical Cationic and Anionic Photopolymerization. 2nd edition. Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. p. 287-294.

Specific examples of photoinitiators may include, but are not limited to, the following compounds or combinations thereof: benzophenone and substituted benzophenones, 1-hydroxycyclohexyl phenyl ketone, thioxanthones such as isopropylthioxanthone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl) butan-1-one, benzyl dimethylketal, bis (2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide, 2,4,6 trimethylbenzoyldiphenylphosphine oxide, 2,4,6-trimethoxybenzoyldiphenylphosphine oxide, 2-methyl-1-[4-(methylthio) phenyl]-2-morpholinopropan-1-one, 2,2-dimethoxy-1,2-diphenylethan-1-one or 5,7-diiodo-3-butoxy-6-fluorone.

Suitable commercial photoinitiators include Irgacure™ 184, Irgacure™ 500, Irgacure™ 369, Irgacure™ 1700, Irgacure™ 651, Irgacure™ 819, Irgacure™ 1000, Irgacure™ 1300, Irgacure™ 1870, Darocur™ 1173, Darocur™ 2959, Darocur™ 4265 and Darocur™ ITX available from CIBA SPECIALTY CHEMICALS, Lucerin™ TPO available from BASF AG, Esacure™ KT046, Esacure™ KIP150, Esacure™ KT37 and Esacure™ EDB available from LAMBERTI, H-Nu™ 470 and H-Nu™ 470X available from SPECTRA GROUP Ltd.

The photoinitiator may be a so-called diffusion hindered photoinitiator. A diffusion hindered photoinitiator is a photoinitiator which exhibits a much lower mobility in a cured ink layer than a monofunctional photoinitiator, such as benzophenone. Several methods can be used to lower the mobility of the photoinitiator. One way is to increase the molecular weight of the photoinitiators so that the diffusion speed is reduced, e.g. polymeric photoinitiators. Another way is to increase its reactivity so that it is built into the polymerizing network, e.g. multifunctional photoinitiators (having 2, 3 or more photoinitiating groups) and polymerizable photoinitiators.

The diffusion hindered photoinitiator for the radiation curable inkjet is preferably selected from the group consisting of non-polymeric multifunctional photoinitiators, oligomeric or polymeric photoinitiators and polymerizable photoinitiators. Most preferably the diffusion hindered photoinitiator is a polymerizable initiator or a polymeric photoinitiator.

A preferred diffusion hindered photoinitiator contains one or more photoinitiating functional groups derived from a Norrish type I-photoinitiator selected from the group consisting of benzoinethers, benzil ketals, α,α-dialkoxyacetophenones, α-hydroxyalkylphenones, α-aminoalkylphenones, acylphosphine oxides, acylphosphine sulphides, α-haloketones, α-halosulfones and phenylglyoxalates.

A preferred diffusion hindered photoinitiator contains one or more photoinitiating functional groups derived from a Norrish type II-initiator selected from the group consisting of benzophenones, thioxanthones, 1,2-diketones and anthraquinones.

Suitable diffusion hindered photoinitiators are also those disclosed in EP-A 2065362 in paragraphs [0074] and [0075]for difunctional and multifunctional photoinitiators, in paragraphs [0077] to [0080] for polymeric photoinitiators and in paragraphs [0081] to [0083] for polymerizable photoinitiators.

A preferred amount of photoinitiator is 0.1-20 wt %, more preferably 2-15 wt %, and most preferably 3-10 wt % of the total weight of the radiation curable inkjet.

In order to increase the photosensitivity further, the radiation curable inkjet may additionally contain co-initiators. Suitable examples of co-initiators can be categorized in three groups: 1) tertiary aliphatic amines such as methyldiethanolamine, dimethylethanolamine, triethanolamine, triethylamine and N-methylmorpholine; (2) aromatic amines such as amylparadimethyl-aminobenzoate, 2-n-butoxyethyl-4-(dimethylamino) benzoate, 2-(dimethylamino)-ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and 2-ethylhexyl-4-(dimethylamino)benzoate; and (3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates (e.g., diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates (e.g., N-morpholinoethyl-acrylate). The preferred co-initiators are aminobenzoates.

When one or more co-initiators are included into the radiation curable inkjet ink, preferably these co-initiators are diffusion hindered for safety reasons.

A diffusion hindered co-initiator is preferably selected from the group consisting of non-polymeric di- or multifunctional co-initiators, oligomeric or polymeric co-initiators and polymerizable co-initiators. More preferably the diffusion hindered co-initiator is selected from the group consisting of polymeric co-initiators and polymerizable co-initiators. Most preferably the diffusion hindered co-initiator is a polymerizable co-initiator having at least one (meth)acrylate group, more preferably having at least one acrylate group.

The radiation curable solder mask inkjet ink preferably includes a polymerizable or polymeric tertiary amine co-initiator.

Preferred diffusion hindered co-initiators are the polymerizable co-initiators disclosed in EP-A 2053101 in paragraphs [0088] and [0097].

The radiation curable solder mask inkjet ink preferably includes the (diffusion hindered) co-initiator in an amount of 0.1 to 20 wt %, more preferably in an amount of 0.5 to 15 wt %, most preferably in an amount of 1 to 10 wt % of the total weight of the radiation curable inkjet ink.

Polymerization Inhibitors

The radiation curable solder mask inkjet ink may contain at least one inhibitor for improving the thermal stability of the ink.

Suitable polymerization inhibitors include phenol type antioxidants, hindered amine light stabilizers, phosphor type antioxidants, hydroquinone monomethyl ether commonly used in (meth)acrylate monomers, and hydroquinone, t-butylcatechol, pyrogallol, 2,6-di-tert.butyl-4-methylphenol (=BHT) may also be used.

Suitable commercial inhibitors are, for example, Sumilizer™ GA-80, Sumilizer™ GM and Sumilizer™ GS produced by Sumitomo Chemical Co. Ltd.; Genorad™ 16, Genorad™18 and Genorad™ 20 from Rahn AG; Irgastab™ UV10 and Irgastab™ UV22, Tinuvin™ 460 and CGS20 from Ciba Specialty Chemicals; Floorstab™ UV range (UV-1, UV-2, UV-5 and UV-8) from Kromachem Ltd, Additol™ S range (S100, S110, S120 and S130) from Cytec Surface Specialties.

The inhibitor is preferably a polymerizable inhibitor.

Since excessive addition of these polymerization inhibitors may lower the curing speed, it is preferred that the amount capable of preventing polymerization is determined prior to blending. The amount of a polymerization inhibitor is preferably lower than 5 wt %, more preferably lower than 3 wt % of the total radiation curable inkjet ink.

Surfactants

The radiation curable solder mask inkjet ink may contain at least one surfactant, but preferably no surfactant is present. If no surfactant is present, the radiation curable inkjet ink does not spread well allowing the generation of thin lines.

The surfactant can be anionic, cationic, non-ionic, or zwitter-ionic and is usually added in a total quantity less than 1 wt % based on the total weight of the radiation curable inkjet ink.

Suitable surfactants include fluorinated surfactants, fatty acid salts, ester salts of a higher alcohol, alkylbenzene sulfonate salts, sulfosuccinate ester salts and phosphate ester salts of a higher alcohol (for example, sodium dodecylbenzenesulfonate and sodium dioctylsulfosuccinate), ethylene oxide adducts of a higher alcohol, ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of a polyhydric alcohol fatty acid ester, and acetylene glycol and ethylene oxide adducts thereof (for example, polyoxyethylene nonylphenyl ether, and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS & CHEMICALS INC.).

Preferred surfactants are selected from fluoric surfactants (such as fluorinated hydrocarbons) and silicone surfactants. The silicone surfactants are preferably siloxanes and can be alkoxylated, polyether modified, polyether modified hydroxy functional, amine modified, epoxy modified and other modifications or combinations thereof. Preferred siloxanes are polymeric, for example polydimethylsiloxanes.

Preferred commercial silicone surfactants include BYK™ 333 and BYK™ UV3510 from BYK Chemie.

In a preferred embodiment, the surfactant is a polymerizable compound.

Preferred polymerizable silicone surfactants include a (meth)acrylated silicone surfactant. Most preferably the (meth)acrylated silicone surfactant is an acrylated silicone surfactant, because acrylates are more reactive than methacrylates.

In a preferred embodiment, the (meth)acrylated silicone surfactant is a polyether modified (meth)acrylated polydimethylsiloxane or a polyester modified (meth)acrylated polydimethylsiloxane.

Preferably the surfactant is present in the radiation curable inkjet ink in an amount of 0 to 3 wt % based on the total weight of the radiation curable inkjet ink.

Flame Retardant

The radiation curable solder mask inkjet ink may further contain a flame retardant. In principle all known flame retardants may be used. However, the flame retardant is preferably not a halogen containing compound.

Preferred flame retardants are inorganic flame retardants, such as Alumina Trihydrate and Boehmite, and organo-phosphor compounds, such as organo-phosphates (e.g. triphenyl phosphate (TPP), resorcinol bis (diphenylphosphate) (RDP), bisphenol A diphenyl phosphate (BADP), and tricresyl phosphate (TCP)); organo-phosphonates (e.g. dimethyl methylphosphonate (DMMP)); and organo-phosphinates (e.g. aluminium diethyl phosphinate).

Other preferred organo-phosphor compounds are disclosed in U.S. Pat. No. 8,273,805.

Preparation of Inkjet Inks

The preparation of pigmented radiation curable inkjet inks is well-known to the skilled person. Preferred methods of preparation are disclosed in paragraphs [0076] to [0085] of WO2011/069943.

Inkjet Printing Devices

The radiation curable solder mask inkjet ink may be jetted by one or more print heads ejecting small droplets in a controlled manner through nozzles onto a substrate, which is moving relative to the print head(s).

A preferred print head for the inkjet printing system is a piezoelectric head. Piezoelectric inkjet printing is based on the movement of a piezoelectric ceramic transducer when a voltage is applied thereto. The application of a voltage changes the shape of the piezoelectric ceramic transducer in the print head creating a void, which is then filled with ink. When the voltage is again removed, the ceramic expands to its original shape, ejecting a drop of ink from the print head. However the inkjet printing method according to the present invention is not restricted to piezoelectric inkjet printing. Other inkjet print heads can be used and include various types, such as a continuous type.

The inkjet print head normally scans back and forth in a transversal direction across the moving ink-receiver surface. Often the inkjet print head does not print on the way back. Bi-directional printing is preferred for obtaining a high areal throughput. Another preferred printing method is by a “single pass printing process”, which can be performed by using page wide inkjet print heads or multiple staggered inkjet print heads which cover the entire width of the ink-receiver surface. In a single pass printing process the inkjet print heads usually remain stationary and the ink-receiver surface is transported under the inkjet print heads.

Curing Devices

The radiation curable solder mask inkjet ink can be cured by exposing them to actinic radiation, such as electron beam or ultraviolet radiation. Preferably the radiation curable inkjet ink is cured by ultraviolet radiation, more preferably using UV LED curing.

In inkjet printing, the curing means may be arranged in combination with the print head of the inkjet printer, travelling therewith so that the curable liquid is exposed to curing radiation very shortly after been jetted.

In such an arrangement, with the exception of UV LEDs, it can be difficult to provide a small enough radiation source connected to and travelling with the print head. Therefore, a static fixed radiation source may be employed, e.g. a source of curing UV-light, connected to the radiation source by means of flexible radiation conductive means such as a fibre optic bundle or an internally reflective flexible tube.

Alternatively, the actinic radiation may be supplied from a fixed source to the radiation head by an arrangement of mirrors including a mirror upon the radiation head.

The source of radiation may also be an elongated radiation source extending transversely across the substrate to be cured. It may be adjacent the transverse path of the print head so that the subsequent rows of images formed by the print head are passed, stepwise or continually, beneath that radiation source.

Any ultraviolet light source, as long as part of the emitted light can be absorbed by the photo-initiator or photo-initiator system, may be employed as a radiation source, such as, a high or low pressure mercury lamp, a cold cathode tube, a black light, an ultraviolet LED, an ultraviolet laser, and a flash light. Of these, the preferred source is one exhibiting a relatively long wavelength UV-contribution having a dominant wavelength of 300-400 nm. Specifically, a UV-A light source is preferred due to the reduced light scattering therewith resulting in more efficient interior curing.

UV radiation is generally classed as UV-A, UV-B, and UV-C as follows:

UV-A: 400 nm to 320 nm

UV-B: 320 nm to 290 nm

UV-C: 290 nm to 100 nm.

In a preferred embodiment, the radiation curable inkjet ink is cured by UV LEDs. The inkjet printing device preferably contains one or more UV LEDs preferably with a wavelength larger than 360 nm, preferably one or more UV LEDs with a wavelength larger than 380 nm, and most preferably UV LEDs with a wavelength of about 395 nm.

Furthermore, it is possible to cure the ink image using, consecutively or simultaneously, two light sources of differing wavelength or illuminance. For example, the first UV-source can be selected to be rich in UV-C, in particular in the range of 260 nm-200 nm. The second UV-source can then be rich in UV-A, e.g. a gallium-doped lamp, or a different lamp high in both UV-A and UV-B. The use of two UV-sources has been found to have advantages e.g. a fast curing speed and a high curing degree.

For facilitating curing, the inkjet printing device often includes one or more oxygen depletion units. The oxygen depletion units place a blanket of nitrogen or other relatively inert gas (e.g. CO₂), with adjustable position and adjustable inert gas concentration, in order to reduce the oxygen concentration in the curing environment. Residual oxygen levels are usually maintained as low as 200 ppm, but are generally in the range of 200 ppm to 1200 ppm.

EXAMPLES Materials

All materials used in the following examples were readily available from standard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS (Belgium) unless otherwise specified. The water used was deionized water.

SR606A is neopentylglycol hydroxypivalate diacrylate available as Sartomer™ SR606A from ARKEMA.

ITX is Speedcure™ ITX, a mixture of isopropyl thioxanthone isomers, from LAMBSON SPECIALTY CHEMICALS.

TPO is 2,4,6-trimethylbenzoyldiphenylphosphine oxide, supplied by RAHN AG.

EPD is ethyl-4-(dimethylamino)benzoate, available under the trade name of Genocure™ EPD from RAHN AG.

BAPO is a bis(2,4,6-trimethylbenzoyl)-phenylphoshineoxide photoinitiator available as Irgacure™ 819 from BASF.

INHIB is a mixture forming a polymerization inhibitor having a composition according to Table 2.

TABLE 2 Component wt % DPGDA 82.4 p-methoxyphenol 4.0 2,6-di-tert-butyl-4- 10.0 methylphenol Cupferron ™ AL 3.6

Cupferron™ AL is aluminum N-nitrosophenylhydroxylamine from WAKO CHEMICALS LTD.

Ebecryl 1360 AK is a polysiloxane hexa acrylate slip agent from ALLNEX.

IBOA is isobornylacrylate, available as Sartomer™ 506D from ARKEMA.

DPGDA is dipropylenediacrylate, available as Sartomer SR508 from ARKEMA.

TMPTA is trimethylol propane triacrylate, available as Sartomer™ SR351 from ARKEMA.

FST426R is a difunctional urethane methacrylate from CLARIANT.

VEEA is 2-(vinylethoxy)ethyl acrylate available from NIPPON SHOKUBAI, Japan.

Cyan is SUN FAST BLUE 15:4, a cyan pigment available from SUN CHEMICALS.

Yellow is CROMOPHTAL YELLOW D 1085J, a yellow pigment from BASF.

Green is IRGALITE GREEN 6G, a green pigment from BASF.

Disperbyk 162 is a dispersing agent and has been precipitated from a solution available from BYK (ALTANA).

Solsperse 35000 is a polymeric dispersant from LUBRIZOL.

Solsperse 12000 is a polymeric dispersant from LUBRIZOL.

HDDA is 1,6-hexanedioldiacrylate from ALDRICH.

Methods Coatings/Prints of the Solder Mask Inkjet Inks

To evaluate the adhesion and the solder resistance of the solder mask inkjet inks, the inks were coated on a brushed copper foil (35μ) at a coating thickness of 20μ and cured using a H-bulb (1 pass at 20 m/min). Additionally, the coatings were thermally cured at 150° C. during 30 minutes.

Evaluation Solder Resistance

The solder resistance of the solder mask inkjet inks was evaluated using a SPL600240 Digital Dynamic Solder Pot available from L&M PRODUCTS filled with a “K” Grade 63:37 tin/lead solder available from SOLDER CONNECTION. The temperature of the solder was set at 290° C.

Using a Q-tip, a solder flux SC7560A from SOLDER CONNECTION was applied on the surface of the samples (i.e. coatings of the solder mask inkjet ink on a copper surface) to clean the surface. The solder flux was dried by placing the samples for 10 minutes above the solder pot.

After placing the sample in the solder pot, a solder wave was created for 10 seconds after which the samples were cooled for at least 10 minutes.

The adhesion of the solder mask inkjet inks on the copper surface was then evaluated with a tape test on the cooled samples. A black tape Tesa 4104/04 from TESA AG, Germany was taped onto the coating and the tape was removed immediately thereafter by hand.

A visual evaluation resulted in an adhesion quality ranging from 0 (very good adhesion) to 5 (very poor adhesion).

Adhesion

The adhesion was evaluated by a cross-cut test according to ISO2409:1992(E). Paints (International standard 1992-08-15) using a Braive No. 1536 Cross Cut Tester from BRAIVE INSTRUMENTS with spacing of a 1 mm between cuts and using a weight of 600 g, in combination with a Tesatape™ 4104 PVC tape. The evaluation was made in accordance with a criterion described in Table 3, where both the adhesion in the cross-cut and outside the cross-cut were evaluated.

TABLE 3 Evaluation value Criterion 0 Nothing removed, perfect adhesion. 1 Detachment of only very small parts of the cured layer, almost perfect adhesion. 2 Minor parts of the cured layer was removed by the tape, good adhesion 3 Parts of the cured layer were removed by the tape, poor adhesion. 4 Most of the cured layer was removed by the tape, poor adhesion. 5 The cured layer was completely removed from the substrate by the tape, no adhesion.

Viscosity

The viscosity of the inks was measured at 45° C. and at a shear rate of 1 000 s⁻¹ using a “Robotic Viscometer Type VISCObot” from CAMBRIDGE APPLIED SYSTEMS.

For industrial inkjet printing, the viscosity at 45° C. and at a shear rate of 1000 s⁻¹ is preferably between 3 and 20 mPa·s. More preferably the viscosity at 45° C. and at a shear rate of 1000 s⁻¹ is less than 15 mPa·s.

Preparation of the Adhesion Promoters Preparation of Adhesion Promoter RAFT-1

RAFT-1, 4-(benzenecarbonothioylsulfanyl)-4-cyano-pentanoic acid was supplied by Adrich.

Preparation of Adhesion Promoter RAFT-2

RAFT-2 (l-cyano-1-methyl-propyl) benzenecarbodithiolate) was prepared according to the following procedure.

Bis(thiobenzoyl)disulfide

86.4 g (2.70 mol) elementary sulfur was added to a freshly prepared sodium methanolate solution, prepared by adding 31 g (1.35 mol) sodium to 770 ml dry methanol. 77 ml (85 g, 0.672 mol) benzyl chloride (HCl free, propylene oxide stabilized) was added to the mixture over one hour while keeping the mixture under an argon atmosphere. During the addition the temperature rose to 45° C. The mixture was then refluxed for another 6 hours. After cooling the mixture to 0° C., 700 ml of a 2N solution of hydrochloric acid, precooled to 0° C., was added. A red oily layer was formed. The isolated layer was treated with a mixture of 500 ml of a 2N sodium hydroxide solution and 150 ml t.butyl methyl ether. The excess of sulfur was removed and the aqueous layer isolated. The aqueous layer was extracted twice with 150 ml t-butyl methyl ether. The aqueous layer was isolated and 1350 ml of t.butyl methyl ether was added. A solution of 56 g (0.17 mol) K₃Fe(CN)₆ in 1350 ml water was added while vigorously stirring. The organic layer was isolated, washed twice with 500 ml water and dried over MgSO₄. The solvent was removed under reduced pressure and 14.3 g of the crude bis(thiobenzoyl)disulfide was isolated as a red oil. The crude bis(thiobenzoyl)disulfide was directly used without further purification.

(1-cyano-1-methyl-propyl) benzenecarbodithiolate

8.9 g of the crude bis(thiobenzoyl)disulfide was dissolved in 170 ml ethyl acetate. 2.63 g (13.7 mol) 2,2′-azobis(2-methylbutyronitrile) was added and the mixture was refluxed for 7 hours under argon atmosphere. The solvent was removed under reduced pressure and the crude (1-cyano-1-methyl-propyl) benzenecarbodithiolate was purified on Merck-Kieselgel 60, using heptane/t.butyl methyl ether 9/1 as eluent. 3.05 g of (1-cyano-1-methyl-propyl) benzenecarbodithiolate was isolated.

1-cyano-1-methyl-propyl) benzenecarbodithiolate was analyzed using ¹H-NMR spectroscopy (CDCL₃/TMS as reference: 1.11 ppm (t, 3H), 1.92 ppm (s, 3H), 2.1 ppm (m, 1H), 2.24 ppm (m, 1H), 7.39 ppm (m, 2H), 7.55 ppm (m, 1H), 7.9 ppm (m, 2H)).

Preparation of Adhesion Promoter RAFT-3

RAFT-3 (2-(1-carboxy-1-methyl-ethyl)sulfanylcarbothioylsulfanyl-2-methyl-propanoic acid) was prepared according to Haraguchi et al. (Biomacromolecules, 15(6), 1992-2003 (2014)).

Preparation of Adhesion Promoter RAFT-4

RAFT-4 (2-ethoxycarbothioylsulfanyl-2-methyl-propanoic acid) was prepared according to the following procedure.

35 g (0.218 mol) of ethylxanthogenate potassium salt was added to 50 ml water. The mixture was cooled to 5° C. and 16.25 g (0.0973 mol) 2-bromo-2-methyl-propionic acid was added. The reaction was allowed to continue for 72 hours at room temperature. The reaction mixture was extracted twice with 75 ml t.butyl methyl ether. The aqueous layer was isolated and the pH of the aqueous layer was adjusted to pH 2, using a concentrated hydrochloric acid solution. Small residues of t.butyl methyl ether were removed under reduced pressure and 2-ethoxycarbothioylsulfanyl-2-methyl-propanoic acid crystallized from the medium. 2-ethoxycarbothioylsulfanyl-2-methyl-propanoic acid was isolated by filtration.

10.7 g of 2-ethoxycarbothioylsulfanyl-2-methyl-propanoic acid was isolated (m.p. 104° C.) and analyzed using ¹H-NMR spectroscopy (DMSO d6:1.34 ppm (t, 3H), 1.52 ppm (s, 6H), 4.57 ppm (q, 2H)).

Preparation of Adhesion Promoter RAFT-17

RAFT-17 (2-(diethylcarbamothioylsulfanyl)-2-methyl-propanoic acid was prepared according to the following procedure.

10.2 g (0.14 mol) diethylamine, 81.3 g (1.4 mol) acetone, 2.9 g (7.28 mmol) Aliquat 336 and 19.8 g water were mixed. The mixture was cooled to 0° C. and 10.4 g (0.136 mol) carbon disulfide was added over one hour. The mixture was stirred for 30 minutes while keeping the temperature below 10° C. 21.7 g (0.182 mol) chloroform was added over 30 minutes, followed by the addition of 52.3 ml 10 N sodium hydroxide over 30 minutes, while keeping the temperature below 25° C. The reaction was allowed to continue for 16 hours while keeping the temperature below 25° C. 60 ml water was added to the mixture and the aqueous layer was isolated. The pH was adjusted to pH=2 using a concentrated hydrochloric acid solution. The mixture was cooled to 0° C. and (2-(diethyl-carbamothioylsulfanyl)-2-methyl-propanoic acid precipitated from the medium. (2-(diethylcarbamothioylsulfanyl)-2-methyl-propanoic acid was isolated by filtration.

26.9 g (2-(diethylcarbamothioylsulfanyl)-2-methyl-propanoic acid was isolated (yield=81.5%) and analyzed using LC-MS on a AmaZon SL mass spectrometer, supplied by BrUker Daltonics. An Altima C18 column (150×3, 5 μm) was used in combination with a Alltima HP CP18 pre-column. A flow rate of 0.5 ml per minute was used at 40° C. A gradient elution was used going from pure water to pure acetonitrile over 13 minutes, followed by an elution in pure acrylonitrile for 17 minutes. A sample of 2 mg (2-(diethylcarbamothioylsulfanyl)-2-methyl-propanoic acid in 20 ml acetonitrile was prepared. 5 μl of this sample was injected. (2-(diethylcarbamothioylsulfanyl)-2-methyl-propanoic acid proved to have a purity of 98.8% based on area percentages.

Preparation of the Cyan and Yellow Pigment Dispersions CPD and YPD

Concentrated Cyan and Yellow and pigment dispersions, respectively CPD and YPD, were prepared having a composition according to Table 4.

TABLE 4 wt % of: CPD YPD Cyan 15 — Yellow — 15 Disperbyk 15 = 162 INHIB  1 = VEEA 69 =

CPD and YPD were prepared as follows: 138 g of 2-(2-vinyloxyethoxy)ethyl acrylate, 2 g of a solution containing 4 wt % of 4-methoxyphenol, 10 wt % of 2,6-Di-tert-butyl-4-methylphenol and 3.6 wt % of Aluminum-N-nitroso phenylhydroxyl amine in dipropylene glycol diacrylate, 200 g of a 30 wt % solution of Disperbyk 162 in 2-(2-Vinyloxyethoxy)ethyl acrylate and 60 g of Cyan (for CPD) or 60 g of Yellow (for YPD) were mixed using a DISPERLUX™ dispenser. Stirring was continued for 30 minutes. The vessel was connected to a NETZSCH MiniZeta mill filled with 900 g of 0.4 mm yttrium stabilized zirconia beads (“high wear resistant zirconia grinding media” from TOSOH Co.). The mixture was circulated over the mill for 120 minutes (residence time of 45 minutes) and a rotation speed in the mill of about 10.4 m/s. During the complete milling procedure the content in the mill was cooled to keep the temperature below 60° C. After milling, the dispersion was discharged into a vessel.

The resulting concentrated pigment dispersions CPD and YPD exhibited an average particle size of respectively 80 nm and 131 nm, as measured with a Malvern™ nano-S, and a viscosity of respectively 51 mPa·s and 114 mPa·s at 25° C. and at a shear rate of 10 s⁻¹.

Preparation of the Green Pigment Dispersions GPD

A concentrated Green pigment dispersion GPD was prepared having a composition according to Table 5.

TABLE 5 wt % of: GPD Green 15 Solsperse 15 35000 Solsperse 1.5 12000 INHIB 1 HDDA 69

330 g of 1,6-Hexanediol diacrylate, 5 g of a solution containing 4 wt % of 4-methoxyphenol, 10 wt % of 2,6-di-tert-butyl-4-methylphenol and 3.6 wt % of Aluminum-N-nitroso phenylhydroxyl amine in dipropylene glycol diacrylate, 500 g of a 30 wt % solution of Solsperse 35000 in 1,6-hexanediol diacrylate, 15 g of Solsperse 12000 and 150 g of Pigment Green 36 (Irgalite Green 6G) were mixed using a DISPERLUX™ dispenser. Stirring was continued for 30 minutes. The vessel was connected to a NETZSCH MiniZeta mill filled with 900 g of 0.4 mm yttrium stabilized zirconia beads (“high wear resistant zirconia grinding media” from TOSOH Co.). The mixture was circulated over the mill for 300 minutes (residence time of 45 minutes) and a rotation speed in the mill of about 10.4 m/s. During the complete milling procedure the content in the mill was cooled to keep the temperature below 60° C. After milling, the dispersion was discharged into a vessel. The resulting concentrated pigment dispersion exhibited an average particle size of 96 nm as measured with a Malvern™ nano-S and a viscosity of 89 mPa·s at 25° C. at a shear rate of 10 s⁻¹.

Example 1

This example illustrates the superior solder mask resistance of UV curable inkjet inks including adhesion promoters according to the present invention.

Preparation of Comparative Ink COMP-1 and Inventive Inks INV-1 to INV-3

The comparative radiation curable inkjet ink COMP-1 and the inventive radiation curable inkjet ink INV-1 to INK-3 were prepared according to Table 6. The weight percentages (wt %) are all based on the total weight of the radiation curable inkjet ink.

TABLE 6 wt % of component COMP-1 INV-1 INV-2 INV-3 GPD 10.00 = = = SR606A 38.00 = = = VEEA 19.75 12.75 = = ACMO 20.00 = = = ITX 4.00 = = = TPO 2.95 = = = EPD 4.00 = = = RAFT-03 —  7.00 — — RAFT-04 — — 7.00 — RAFT-17 — — — 7.00 Ebecryl 0.30 = = = 1360 AK INHIB 1.00 = = =

The adhesion and the solder resistance of the comparative ink COMP-01 and the inventive inks INV-1 to INV-3 were tested as described above. The results are shown in Table 7.

TABLE 7 UV curable ink Solder jet ink Adhesion Resistance COMP-1 5 5 INV-1 1 0 INV-2 0 0 INV-3 — 0

It is clear from the results of Table 7 that the inventive solder mask inkjet inks containing an adhesion promoter according to the present invention all have an improved adhesion towards the Cu-surface and a superior solder resistance compared to a solder mask inkjet ink without such an adhesion promoter.

Example 2

This example illustrates the superior solder mask resistance of UV curable inkjet inks including adhesion promoters according to the present invention.

Preparation of Comparative Ink COMP-2 and Inventive Inks INV-5 and INV-6

The comparative radiation curable inkjet ink COMP-2 and the inventive radiation curable inkjet inks INV-5 and INV-6 were prepared according to Table 8. The weight percentages (wt %) are all based on the total weight of the radiation curable inkjet ink.

TABLE 8 wt % of component COMP-2 INV-5 INV-6 CPD 6.65 = = YPD 6.65 = = TMPTA 5.00 = = SR606 A 33.00 = = VEEA 36.45 31.45 = ITX 4.00 = = TPO 2.95 = = EPD 4.00 = = RAFT-01 —  5.00 — RAFT-02 — — 5.00 Ebecryl 0.10 = = 1360 AK INHIB 1.00 = =

The adhesion and the solder resistance of the comparative ink COMP-02 and the inventive inks INV-5 and INV-06 were tested as described above. The results are shown in Table 9.

TABLE 9 UV curable ink Solder jet ink Adhesion Resistance COMP-2 5 5 INV-5 0 — INV-6 0 0

It is clear from the results of Table 9 that the inventive solder mask inkjet inks containing an adhesion promoter according to the present invention all have an improved adhesion towards the Cu-surface and a superior solder resistance compared to a solder mask inkjet ink without such an adhesion promoter. 

1-15. (canceled)
 16. A radiation curable solder mask inkjet ink comprising: a photoinitiator; a free radical polymerizable compound; and an adhesion promoter; wherein the adhesion promoter has a chemical structure according to Formula I:

wherein R1 is selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, OR5, SR6, and NR7R8; R2 is selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; R3 is selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, and an electron withdrawing group including at least one oxygen carbon double bond or a nitrile group; R2 and R3 may represent atoms that form a 5 to 8 membered ring; R4 is selected from the group consisting of an electron withdrawing group including at least one oxygen carbon double bond or a nitrile group, and a substituted or unsubstituted aryl or heteroaryl group; R5 and R6 are independently selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; R7 and R8 are independently selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; R7 and R8 may represent atoms that form a 5 to 8 membered ring; and none of R1 to R4 includes a photoinitiating moiety.
 17. The radiation curable solder mask inkjet ink according to claim 16, wherein the adhesion promoter has a chemical structure according to Formula II:

wherein R9 is selected from the group consisting of OR12, NR13R14, and a substituted or unsubstituted aryl group; R10 is selected from the group consisting of an ester, an amide, a carboxylic acid, and a nitrile; R11 represents a substituted or unsubstituted alkyl group; R12 is independently selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; R13 and R14 are independently selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; and R13 and R14 may represent atoms that form a 5 to 8 membered ring.
 18. The radiation curable solder mask inkjet ink according to claim 16, wherein the adhesion promoter has a chemical structure according to Formula III:

wherein R15 is selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; and R16 represents a hydrogen atom or a substituted or unsubstituted alkyl group.
 19. The radiation curable solder mask inkjet ink according to claim 16, wherein the adhesion promoter has a chemical structure according to Formula IV:

wherein R17 is selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; R18 is selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, and an electron withdrawing group including at least one oxygen carbon double bond or a nitrile group; R17 and R18 may represent atoms that form a 5 to 8 membered ring; and R19 is selected from the group consisting of an electron withdrawing group including at least one oxygen carbon double bond or a nitrile group, and a substituted or unsubstituted aryl or heteroaryl group.
 20. The radiation curable solder mask inkjet ink according to claim 16, wherein the adhesion promoter has a chemical structure according to Formula V:

wherein R20 and R21 are independently selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group; R20 and R21 may represent atoms that form a 5 to 8 membered ring; R22 and R23 are independently selected from the group consisting of a hydrogen atom and a substituted or unsubstituted alkyl group; R22 and R23 may represent atoms that form a 5 to 8 membered ring; and R24 is selected from the group consisting of a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, and a substituted or unsubstituted aryl or heteroaryl group.
 21. The radiation curable solder mask inkjet ink according to claim 20, wherein R22 and R23 represent an alkyl group including 1 to 4 carbon atoms.
 22. The radiation curable solder mask inkjet ink according to claim 20, wherein R24 is selected from the group consisting of a hydrogen atom and a substituted or unsubstituted alkyl group.
 23. The radiation curable solder mask inkjet ink according to claim 16, wherein an amount of the adhesion promoter is between 0.5 and 20 wt % relative to a total weight of the solder mask inkjet ink.
 24. The radiation curable solder mask inkjet ink according to claim 16, wherein the free radical polymerizable compound is selected from the group consisting of neopentyl glycol hydroxypivalate diacrylate, isobornyl acrylate, dipropylene glycol diacrylate, trimethylol propane triacrylate, and 2-(vinylethoxy)ethyl acrylate.
 25. The radiation curable solder mask inkjet ink according to claim 16, further comprising a cyan pigment, a yellow pigment, or a green pigment.
 26. A method of manufacturing an electronic device, the method comprising: jetting a radiation curable solder mask inkjet ink as defined in claim 16 onto a dielectric substrate including an electrically conductive pattern; and curing the jetted solder mask inkjet ink.
 27. The method according to claim 26, wherein the step of curing is performed using UV radiation.
 28. The method according to claim 26, further comprising: heating the jetted solder mask inkjet ink.
 29. The method according to claim 28, wherein the step of heating is performed at a temperature between 80° C. and 250° C.
 30. The method according to claim 26, wherein the electronic device is a printed circuit board. 